R. Hintsche et al., Microbiosensors Using Electrodes Made in Si-Technology, Frontiers in Biosensorics, Fundamental Aspects, edited by F. W. Scheller et al., Dirk Hauser Verlag, Basel, pp. 267-283, 1997, R. Hintsche et al., Microelectrode arrays and application to biosensing devices, Biosensors & Bioelectronics, Vol. 9, pp. 697-705, 1994, M. Paeschke et al., Voltametric Multichannel Measurements Using Silicon Fabricated Microelectrode Arrays, Electroanalysis, Vol. 7, No. 1, pp. 1-8, 1996, and P. van Gerwen, Nanoscaled Interdigitated Electrode Arrays for Biochemical Sensors, IEEE, International Conference on Solid-State Sensors and Actuators, Chicago, pp. 907-910, Jun. 16-19, 1997, disclose methods for detecting DNA molecules in which biosensors based on electrode arrangements are used for detection.
FIG. 2a and FIG. 2b show such a sensor as is described in R. Hintsche et al. and P. van Gerwen. The sensor 200 has two electrodes 201, 202 made of gold, which are embedded in an insulator layer 203 made of insulator material. Electrode connections 204, 205 are connected to the electrodes 201, 202, and the electrical potential present at the electrode 201, 202 can be fed to said electrode connections. The electrodes 201, 202 are arranged as planar electrodes. DNA probe molecules 206 are immobilized on each electrode 201, 202 (cf. FIG. 2a). The immobilization is effected in accordance with so-called gold-sulfur coupling. The analyte 207 to be examined is applied on the electrodes 201, 202. In this case, the analyte may be for example an electrolytic solution of different DNA sequences.
If the analyte 207 contains DNA strands 208 having a sequence which is complementary to the sequence of the DNA probe molecules 206, then said DNA strands 208 hybridize with the DNA probe molecules 206 (cf. FIG. 2b).
Hybridization of a DNA probe molecule 206 and a DNA strand 208 takes place only when the sequences of the respective DNA probe molecule 206 and of the corresponding DNA strand 208 are complementary to one another. If this is not the case, hybridization does not take place. Consequently, a DNA probe molecule of a predetermined sequence is in each case only able to bind, i.e. hybridize with, a specific DNA strand, namely the DNA strand having a respectively complementary sequence.
If hybridization takes place, then, in addition to other electrical parameters, the capacitance between the electrodes also changes, as can be seen from FIG. 2b. This change in capacitance can be used as a measurement quantity for detecting DNA molecules.
N. L. Thompson, B. C. Lagerholm, Total Internal Reflection Fluorescence: Applications in Cellular Biophysics, Current Opinion in Biotechnology, Vol. 8, pp. 58-64, 1997, discloses a further procedure for examining the electrolyte for the existence of a DNA strand having a predetermined sequence. In this procedure, the DNA strands of the desired sequence are marked with a fluorescent dye and their existence is determined on the basis of the fluorescence properties of the marked molecules. For this purpose, light in the visible or ultraviolet wavelength range is radiated onto the electrolyte and the fluorescent light emitted by the analyte, in particular by the marked DNA strand to be detected, is detected. The fluorescence behavior, i.e. in particular the emitted light beams that are detected, are taken as a basis for determining whether or not the DNA strand having the correspondingly predetermined sequence that is to be detected is contained in the analyte.
This procedure is very complicated since very precise knowledge about the fluorescence behavior of corresponding marker molecule at the DNA strand is necessary and, moreover, a marking reaction of the DNA strands is required before the beginning of the method. Furthermore, a very precise adjustment of the detection means for detecting the emitted light beams is necessary in order that the light beams can actually be detected.
Consequently, this procedure is expensive, complicated and very sensitive to disturbance influences, as a result of which the measurement result can very easily be corrupted.
Moreover, a reduction/oxidation recycling method for detecting macromolecular biopolymers is disclosed in R. Hintsche et al. and M. Paeschke et al.
The reduction/oxidation recycling method, also referred to as redox recycling method hereinafter, is explained in more detail below with reference to FIG. 4a to FIG. 4c. 
FIG. 4a shows a biosensor 400 having a first electrode 401 and a second electrode 402, which are applied on a substrate 403 as insulator layer.
A holding region, configured as a holding layer 404, is applied on the first electrode 401 made of gold. The holding region serves for immobilizing DNA probe molecules 405 on the first electrode 401.
No such holding region is provided on the second electrode.
If the biosensor 400 is intended to be used to detect DNA strands having a sequence which is complementary to the sequence of the DNA probe molecules 405, then the sensor 400 is brought into contact with a solution 406 to be examined in such a way that DNA strands having the complementary sequence that are possibly contained in the solution 406 to be examined can hybridize to the sequence of the DNA probe molecules 405.
FIG. 4b shows the case where the DNA strands 407 to be detected are contained in the solution 406 to be examined and have hybridized to the DNA probe molecules 405.
The DNA strands 407 in the solution to be examined are marked with an enzyme 408, which enables molecules that are described below to be cleaved into partial molecules.
The number of DNA probe molecules 405 provided is usually considerably greater than the number of DNA strands 407 to be determined which are contained in the solution 406 to be examined.
Once the DNA strands 407 with the enzyme 408 that are possibly contained in the solution 406 to be examined have hybridized with the immobilized DNA probe molecules, the biosensor 400 is rinsed, as a result of which the non-hybridized DNA strands are removed and the biosensor 400 is cleaned of the solution 406 to be examined.
An electrically uncharged substance is added to this rinsing solution used for rinsing or a further solution 412 supplied separately in a further phase, said substance containing molecules which can be cleaved into a first partial molecule having a negative first electrical charge and into a second partial molecule having a positive second electrical charge by the enzyme at the hybridized DNA strands 407.
As is shown in FIG. 4c, the negatively charged partial molecules are attracted to the positively charged anode, as is indicated by the arrow 411 in FIG. 4c. 
The negatively charged first partial molecules 410 are oxidized at the first electrode 401, which, as the anode, has a positive electrical potential, and are attracted as oxidized partial molecules 413 to the negatively charged cathode, i.e. the second electrode 402, where they are reduced again.
The reduced partial molecules 414 in turn migrate to the first electrode 401, i.e. to the anode.
An electrical circulating current is generated in this way, this current being approximately proportional to the number of charge carriers respectively generated by the enzymes 408.
The electrical parameter which is evaluated in this method is the change in the electrical current
      ⅆ    I        ⅆ    t  as a function of time t, as is illustrated in the diagram 500 in FIG. 5.
Finally, e.g., WO 00/42217 A1 discloses a method which, as explained in more detail with reference to FIG. 6, is based on the difference in conductivity of single-stranded and double-stranded nucleic acid molecules and therefore resorts to a measurement of an electrical current or of a voltage.
In the method according to WO 00/42217 A1, a single-stranded nucleic acid molecule 602 is immobilized as capture molecule on an electrically conductive sensor surface 601. The capture molecule 602 carries a marking 603, which can emit or take up electrons during a light-inducible redox process.
If light having a suitable wavelength, as symbolized by the arrow 606, is radiated onto the sensor 600, then the redox-active marking 603, excited by the light radiated in, continuously liberates electrons. If the marking is present in a double-stranded hybrid comprising DNA capture molecules 602 and nucleic acids 605 to be detected, this double-stranded hybrid acts as a type of electron pump and conducts electrons, as illustrated by the arrow 607, from the marking 603 to the conductive surface 601, so that a current can be measured at said surface by means of a measuring device 604 (cf. FIGS. 6a, b). However, if no hybridization is effected, single-stranded capture molecules 602 approximately constitute an insulator; consequently, no current flows at the surface 601.
FIG. 6c, corresponding to FIG. 4 from WO 00/42217 A1, shows an illustration which is more precise in molecular detail and in which a photosynthetic bacterial reaction center is used as redox-active marking.
The abovementioned methods for detecting macromolecular biopolymers such as nucleic acids have in common the disadvantage that their detection sensitivity is relatively low, in particular if only small amounts of molecules to be detected are available.
DE 199 40 810 A1 discloses a method and a measuring device for determining a multiplicity of analytes in a sample.
WO 01/75151 A2 discloses a method for detecting macromolecular biopolymers using an electrode arrangement.
DE 199 16 921 A1 discloses an electrical sensor array based on voltametric and/or impedimetric detection principles with serial or parallel read-out.
WO 94/05414 A1 discloses an integrated instrument for manipulation, carrying out of a reaction and detection of samples in microliter to pikoliter dimensions.
U.S. Pat. No. 5,648,213 discloses structures and methods for use in the detection of analytes.